Steer-by-wire systems are increasing in popularity, including in work vehicles such as construction vehicles and farm vehicles. In a steer-by-wire system, the connection between the steering wheel and the wheels and other parts of the steering mechanism is electronic, rather than mechanical. Thus, in order to have a steer-by-wire system, it is necessary to be able to measure the angular displacement of a steering wheel so that the vehicle can turn based upon this displacement since there is no mechanical transmission of the angular displacement of the steering wheel to the vehicle's steering mechanism.
Traditional angular measurement devices have included optical encoders which detect angular displacement based upon a detection of light transmission and/or blockage through a rotating object. Examples of such devices include various disc-shaped objects, such as the following: a circular disc having a thicker ring surrounding a thinner web with holes through the ring; a flat disc with holes proximate a circumferential edge of the disc; a flat piece of glass with scratches etched on its surface to break up the passage of light through the disc, etc. In such configurations, light passes through the holes (or is blocked by the etches) and the angular measurement is determined based upon how frequently light passes through the holes or is blocked by etches in a circumferential position around the disc-shaped object.
However, such system does not permit a determination of direction of rotation since it merely measures frequency of detection of light passing through the holes or apertures (or blockage of light by etches) in the disc-shaped object. Thus, a second sensor in quadrature to the first sensor is provided to determine direction of rotation. Being in quadrature means that they are out of phase 90 degrees. Thus, comparing the outputs of the sensors as a function of time will provide information as to which direction the disc-shaped object is moving since the sensors are fixed at a 90 degree phase shift.
The problems of using typical disc-shaped objects include that the additional sensor in quadrature is needed to determine the direction of motion, as explained above, and also that the disc-shaped objects are not very robust. For example, a slotted disc may be used, and it may be made thin to keep the sensor package as small as possible, which makes it more fragile. These sensor discs can be made from glass, plastic, and sometimes metal. The slots in the disc act as stress concentrators when under load and when heavy shock or vibration loads are applied to the sensor package, which can often lead to failures of the disc itself by causing, for example, cracks, pieces breaking off, etc. As the angular resolution of an encoder using one of these disc-shaped objects is increased by increasing the number of slots around the circumference, the disc becomes even more delicate. In situations where a piece of equipment is used in construction or farming, there are stresses created and other environmental exposures that can damage disc-shaped objects. For example, heavy shock loads and vibration are a problem which can adversely affect disc-shaped objects. The present invention addresses these problems as well as having other advantages, as explained more fully below, with the use of sensors to measure the distance to a cam to determine angular displacement. The present invention can include a cam made from a solid piece of metal or plastic and can easily be designed with minimal stress concentrations which will make the cam itself far more robust to heavy shock and vibration loads than traditional disc-shaped objects.
U.S. Pat. No. 4,942,394 is directed to a Hall Effect Encoder Apparatus. This patent has the sensor on the inside of a rotating shaft rather than on an outside. Thus, the sensor itself is what is moving rather than measuring the movement of a cam. Moreover, the invention is not directed to vehicles.
U.S. patent application Ser. No. 11/446,882 (Published as US 2007/0282507) is directed to Non-Contact Neutral Sensing with Direction Feedback. Here, there is no steer-by-wire system. Furthermore, there is no redundancy. Also, the rotation of a cam with sensors around it does not occur over 360 degrees. The disclosure of this patent application is incorporated by reference herein in its entirety.